This invention relates to a method of manufacturing glass in which raw material is fed as a batch to a continuous glass-melting tank furnace, the batch being melted in a melting zone and passing to a refining zone for the delivery of molten, refined glass.
At the present time, the manufacture of glass on an industrial scale takes place almost exclusively in continuous tank furnaces. The only exceptions to this rule are in the production of glasses of a highly specialised nature, whether by virtue of their composition or their working.
The concept of tank furnaces derives from work done by Friedrich Siemens over a hundred years ago: these furnaces comprise a tank containing molten glass, the tank being surmounted by a superstructure of side walls and a crown which contains the atmosphere above the melt. The heat necessary for melting the raw batch materials to form the glass, an for refining it, is generated by burning gas or fuel oil in that atmosphere. The combustion supporting air is itself preheated by regenerators which recover part of the heat of exhaust gases issuing from the furnace.
In the manufacture of glass, costs are incurred under various heads, in particular capital and maintenance costs of the plant used, raw material costs, labour costs and the fuel consumed in melting and vitrifying the raw materials. Of these, fuel consumption is not the least significant.
The specific fuel consumption will depend on various factors. Economies of scale are possible, so that glass can be produced at lower cost in a larger capacity furnace. In a furnace of given capacity, production will be more economic if the furnace is running at full capacity than if it is producing glass at only a fraction of that rate. The temperature required to form the glass depends on the composition of the batch used to produce that glass, since some raw materials are more difficult to melt than others, and this also affects fuel consumption. The type of glass product to be made from the molten glass can also have an indirect effect on fuel consumption; comparing figured rolled glass or bottle glass with float glass of the same composition, it will be clear that a higher population of optical defects, for example bubbles due to incomplete refining of the glass, can be tolerated in figured or bottle glass than in float glass which should be substantially defect free. In general, glass of higher optical quality requires higher fuel consumption. Finally, mention must be made of fuel consumed in order to maintain the temperature of the furnace in view of heat loss through its walls.
In continuous glass-melting tank furnaces, the vitrifiable batch is fed continuously onto the molten glass at the charging end of the furnace, and it is then melted and refined at very high temperature; the molten glass is then progressively cooled to a temperature appropriate for working. In industrial tank furnaces, melting and refining take place in one compartment of the furnace, whereas temperature adjustment of the refined glass is effected in a second compartment more or less isolated from the first, it being of course understood that there is sufficient continuity for the glass to flow from one end of the furnace to the other.
It was not until several decades after the invention of the continuous tank furnace, until flat glass was being produced widely and on a large scale in the early part of the present century, that glassmakers began to take account of the fact that the bath of molten glass is in continuous movement as a result of quite strong convection currents due to differences in density between glass at different temperatures in different parts of the furnace tank. The currents include relatively cool, so-called return currents flowing along the sole of the tank, and hotter currents flowing at the surface. The return currents flow from cooler regions of the furnace towards its hottest point (the "hot spot"), while the surface currents flow away from the hot spot. The convection currents give rise to an appreciable increase in the consumption of heat energy in the furnace because there is a continuous recirculating flow of glass which is cyclically cooled at the side walls of the furnace and reheated at the hot spot: the glass acts to carry a continuous flux of heat energy which is lost through the side walls of the furnace.
Some of those skilled in the art believe that these convection currents have a favourable effect on the melting and refining of the glass inter alia by promoting homogenization of the melt: others object that these currents can have a disadvantageous effect because they ensure the dispersion of accidental defects in the melt, and because they can dehomogenize the glass if their flow pattern is unsuitable. It is agreed, however, that return currents which circulate from one zone of the furnace tank to another will inevitably be present. Indeed it is also agreed that for the maintenance of high quality in the production of some types of glass, for example flat glass, the presence of return currents flowing from one zone of the furnace to another is essential.
As a result, in an attempt to control these currents, glassmakers have adopted certain measures intended to modify the strength and distribution of these convection currents. Among other things, it has been proposed to place obstacles such as bridges or floaters and sills in the path of these currents to guide their circulation. It has also been proposed to modify the plan or horizontal cross section of the furnace in order to provide necks to brake and concentrate these currents.
An early example of such a proposal is set out in British Patent Specification No 250,536 (Societe Anonyme des Manufactures des Glaces et Produits Chimiques de St Gobain, Chauny et Cirey). That specification proposes dividing the tank furnace into separate melting and refining compartments in such a way that glass leaves the melting tank at its base and flows upwards through a passageway into the refining compartment. The glass flows over a sill in the upstream end of the refining compartment and then into a into a deeper downstream portion of the refining compartment. The molten glass is then led off from the base of the refining compartment and passed to what is called a pouring zone. The object of that invention is to promote rapid refining of the glass, and to that end burner ports are provided above the sill in order to heat the glass flowing over the sill where it is shallowest so that refining in fact takes place above the sill where bubbles can escape most easily. It is an essential feature of the process there described that the glass should be at its hottest over the sill and should cool as it enters the deeper downstream portion of the refining compartment so that it can be drawn off from the base of that compartment. If the glass flowing in a thin layer over the sill is heated very strongly, as it must be according to that proposal, it necessarily follows that the refractory material of which the sill is made will also be heated very strongly with a consequent high risk of severe sill erosion. It accordingly becomes necessary to cool the sill. As a result, heat energy is removed from the furnace so that fuel is wasted. In addition, with the constructions of tank furnace illustrated in that specification there will be a large energy loss due to contact of the molten glass with a large area of furnace wall which is exposed to the atmosphere as the molten glass flows from the melting compartment to the refining compartment. A further disadvantage of this proposal is that because the glass is refined where it is very shallow and is then allowed to cool as it flows down into a deeper portion of the refining compartment, there will be no substantial circulating currents set up in the refining compartment with the result that homogenization of the glass will be poor.
Other proposals have been made along similar lines, but none has been found commercially acceptable because of the high specific fuel consumption required to produce glass of a satisfactory quality, and further research and experimentation has led to the proposal of a furnace which is of a very different design.
The possibility has been studied of effecting melting in a vertical column down which the vitrifiable batch falls against rising exhaust fumes and flames generated at the base of the column; the glass melted in this way is then refined in a tank specially constructed for that purpose. In fact such a system can suffer from unacceptable refractory erosion at the base of the melting column, and so this also has not been adopted commercially. Although certain proposals would in theory allow production of glass with a low specific fuel consumption, they are surrounded by practical difficulties which stand in the way of their commercial adoption.
The low fuel efficiency of tank furnaces has been known for a long time, but it has become of particular importance since the oil crisis of the early nineteen-seventies. Efforts have, however, been concentrated on apparatus peripheral to the tank rather than on the tank itself. Attempts have been made to generate a gas-fed flame which is more radiant, to improve heat recovery for example by using regenerator flue gases to preheat the vitrifiable batch, and to increase furnace insulation. But even if these steps do give an increased specific yield of glass in relation to heat energy consumed, they do not have any intrinsic effect on the nature of the glass forming process: they have no effect on the basic cause of heat loss from the melt due in part to the recirculating return currents: they treat the symptoms, not the disease.